1. Field of the Invention
This invention relates generally to a spinal rod fixation apparatus having an elongate rod, a vertebral body screw, and a stabilizing staple element, and more particularly to a rod, screw and staple assembly having a rotatable screw head which is selectively lockable in combination with the rod and staple to provide enhanced stability and bone holding strength.
2. Description of the Prior Art
The spinal column is highly complex system of bones and connective tissues which houses and protects critical elements of the nervous system and the arterial and venous bodies in close proximity thereto. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist through a wide range of motion.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of mechanical implant structures have been disclosed in the art which are used during surgical intervention to immobilize segments of the spine which are either unstable or have, in combination, become so irregular that they threaten the continued health of the patient. These assemblies are generally classified as anterior, posterior, or lateral. As the classifications suggest, posterior implants are attached to the back of the spinal column, generally hooking under the lamina and entering into the central canal, attaching to the transverse process, or coupling through the pedicle bone using pedicle screws. Posterior fixation assemblies using such screws are generally used in short sequence immobilization indications, and generally in the larger, lower lumbar bones, for their attending pathologies. Lateral and anterior assemblies, by contrast are coupled to the anterior portion of the spine, which is the sequence of vertebral bodies, and are often used throughout long segments of lumbar and thoracic sequences of vertebrae. A specific pathology which often requires significant surgical intervention along extended numbers of vertebrae is scoliosis. The present invention relates to spinal fixation devices for immobilizing and altering the alignment of the spine over a large number, for example more than three or four, vertebra by means of affixing at least one elongate rod to the sequence of selected vertebral bodies.
Anterior (and/or lateral) "rod assemblies" of the prior art have generally been inserted into the bone either unicortically or bicortically, wherein the shaft of the screw transects (and gains fixation strength as it passes through) one or two exterior layers of the vertebral bone, respectively. Exposing the tip of the screw shaft through the opposing side of the bone's exterior surface does, however, entail a risk inasmuch as important blood vessels, nerve roots, as well as other critical tissues are often in jeopardy of injury through contact with an exposed screw tip. Bicortical fixation, however, provides greatly enhanced strength against pullout; an event in which the screw is pulled free of the bone as its grip inside the vertebra fails to hold.
In order to provide enhanced stability against such pullout events, a staple as shown in FIG. 1, was designed. The basic staple of the prior art comprises a flat metal surface 10 having a hole 12 formed in the center thereof. The corners 14 of the staple 10 are curved downwardly to form four spaced apart spikes. The basic vertebral body screw 20, rod 30 and top locking nut 40 of the prior art are shown in FIG. 2, in conjunction with the staple 10, in an exploded assembly diagram. The screw 20 is inserted through the hole 12 in the staple 10 until the wider top, rod receiving portion 22 of the screw, contacts and seats in the hole 12 of the staple. The wider base, provided by the staple 10, impairs toggling action by the screw within the bone, and is intended to prevent motion which can cause the screw to bone interface from breaking down. The rod 30 is then placed in the rod receiving channel 24 of the screw head 22, and a top locking nut 40 is advanced onto the top of the screw head 22, thereby locking the rod to the screw 20, and by association, to the bone.
In some advanced embodiments of this screw and staple design (not shown), the hole and the bottom of the screw are designed such that the screw may be inserted at a modest angle to the staple, thus permitting stable seating of the screw and staple, despite slight offsets of the screw relative to the bone surface.
These screw and staple assemblies of the prior are, however, do not prevent the most frequent pullout failure mechanism, which is direct vertical force pullout which is caused when the rod itself imparts a sufficient stress against the shaft to cause the screw to back out of the hole. In addition, the ability of the staple to impair toggling of the screw in the bone is limited insofar as the screw and staple are not held together by any specific means, and therefore does not prevent the screw from rotating in the hole and causing microfractures, which can lead to bone failure. Further, the prior art designs limit the ability of the rod receiving head of the screw to be properly aligned with the rod. In many instances, the screw is not fully seated in the hole of the staple because the screw had to be backed out of the hole by the surgeon to align the rod in the rod receiving channel of the head. In such fixed head screw designs, the alignment of the rod receiving channel of the head is not independent of the height of the screw insofar as the rotation of the head causes the threading of the screw to rise up from or dig deeper into the bone. This creates a substantial difficulty for surgeons as they try to seat the rod properly into the screw head while simultaneously having the screw head seat in the hole in the staple.
It is, therefore, the principal object of the present invention to provide a vertebral body screw, rod, and staple assembly which provides enhanced stability and pullout protection.
In addition, it is an object of the present invention to provide such an assembly which includes a stable locking of the staple to the screw so that the screw head can be positioned in the ideal orientation without risking the union of the screw and staple.
It is a related object of the present invention to introduce a screw having a head which may be independently rotated relative to the shaft of the screw, but which may be securely locked in the desired orientation when combined with the screw and staple.
Accordingly it is also an object of the present invention to provide an assembly in which the staple and screw are lockably coupled together upon completion of the implantation
It is also a principal object of the present invention to provide a reliable, durable, and efficient, long term fixation assembly for spine stabilization.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.